The Autocannon is a direct-fire ballistic weapon, firing HEAP (High-Explosive Armor-Piercing) rounds at targets either singly or in bursts.

Different manufacturers and models of autocannons have different calibers (25mm-203mm) and rates of fire. Due to this, autocannons are grouped into generic "classes" of autocannons with common damage ratings, with Autocannon/20s doing massive damage while having very short range.[2]

This autocannon class can decapitate any 'Mech with one good hit to the head, regardless of the weight class. The head of any 'Mech cannot mount more than a little more than a tons worth of standard armor. The AC/20 is capable of ripping nearly a ton and a quarter of armor off a 'Mech in any given volley (or shot, based on the model). This is enough damage to take down even the heaviest of 'Mechs if aimed well. Only Hardened Armor or Ballistic-Reinforced Armor can save a MechWarrior from a hit from this class of cannon to the head, if the full value of armor is mounted on the head of the 'Mech.

An example of the rating system: the Crusher Super Heavy Cannon is a 150mm weapon firing ten shells per "round" while the Chemjet Gun is a 185mm weapon firing much slower, and causing higher damage per shell. Despite their differences, both are classified as Autocannon/20s due to their damage output.

Caseless Autocannon/20: A smaller and lighter version of the Autocannon/20 specifically designed to use caseless ammunition, giving it three times the ammunition capacity per given mass, along with a size and weight profile just slightly larger than the Autocannon/10. However, it has one critical design flaw that led to it being phased out in favor of a more modern variant of Caseless munitions; though these newer munitions are not as lightweight as their counterparts, they can be used with a simple modification to existing Autocannons and are far less prone to explosions in the event of a weapon jam.